Metodología

The site at which lambs were reared impacted on weight throughout life. The differences between sites are likely to be partly explained by nutrition and dam genetics. Trangie had consistently high lamb weights and Kirby consistently low lamb weights, possibly indicating increased and reduced planes of nutrition respectively. However, the Information Nucleus Flock was not designed to control nutrition and thus conclusions regarding the nutritional impact on birth weight are difficult to assess. As dam genetics vary at each site they may also contribute to the variation in weights between sites. If dam genetics at each site were entirely responsible for differences in lamb weight there would be minimal differences in lamb weights at each site across different years however lamb weight at a single site varied between years by nearly 40% of the average lamb weight. Thus while dam genetics varies between sites and contributes to lamb weight differences they are not solely responsible for these differences.

115 The year in which lambs were born had no impact at birth and was relatively consistent between 100 and 240 days. The effect was small being as little as 10% of the magnitude of the site of rearing, which had the largest effect. While the effect was small it may be due to post weaning nutritional availability within a year which would explain why the impact is not present at birth. However, the sites at which lambs were raised were spread across different regions of Australia where pasture availability varied dramatically. Therefore it is likely that there are also non nutritional factors which contributed to the impact of lamb weight within a year.

Male lambs were heavier and grew faster at all times measured and this has been demonstrated in previous studies (Afolayan et al., 2007; Thatcher et al., 1991; Thompson et al., 1985b). Despite male lambs being castrated in this study this effect is consistent with the impact of testosterone, a potent muscle growth stimulant (Lawrence and Fowler, 2002). The study by Thatcher et al. (1991) compared growth rates in rams, wethers and ewes and found that rams grew significantly faster than wethers which in turn grew significantly faster than ewes (P < 0.05). Thus while the wethers in this experiment grew faster than the ewes it can be surmised that growth rate would have been further increased if the male lambs were left entire.

Single born lambs were 22% heavier than twin born lambs and 45% heavier than triplet born lambs at birth which is consistent with findings of Afolayan et al. (2007). The weight differences due to birth type were present throughout life, although they were proportionately less with age. Within a birth type, weight also varied with rearing type with twins raised as singles heavier than twins raised as twins. Triplets raised as singles were also heavier than triplets raised as twins which were heavier than triplets raised as triplets. These weight differences were also present throughout life and reducing throughout age although the weight differences were at most 19%. Thus while both

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birth type and rearing type impact on weight, birth type has the greatest effect. The weight differences at birth were likely to be due to the effects of in-utero nutritional restriction. As birth type represents a prenatal restriction and rear type represents a postnatal to weaning restriction it can be concluded that prenatal restriction has a greater impact on lamb weight. At 100 days the triplet born and raised lambs had the slowest growth rate however by 150 days their growth rate was the fastest suggesting that multiple born lambs experience a period of compensatory growth once nutritional restriction is lifted at weaning. Compensatory growth in immature sheep whose nutrition has been restricted has been previously demonstrated by Thornton et al. (1979). In this experiment the 7 month old sheep (approximately 210 days old) had their nutrition restricted and lost an average of 85g/day over a two month period. When feed was reintroduced ad libitum these sheep then gained 500 g/day in the first 3 days and 300 g/day on average over 33 days to achieve similar live weights to the non-restricted sheep. These growth rates exceed the 114.30 g/day seen in the 150 day old triplet born and raised lambs in this experiment, however the lambs in this experiment were not severely nutritionally restricted post weaning as in the Thornton et al. (1979) experiment which demonstrates the difference between restriction in-utero versus just that imposed during the postnatal period. Interestingly the initial live weight of these Dorset Horn x Merino wethers at 7 months of age was 22.5 kg in the Thornton et al. (1979) experiment, more than 50% lighter than current weight estimations.

Dam age influenced lamb birth weight with lambs born to 2 year old dams being the lightest. This may be due to the utilisation of energy by the 2 year old dam for its own growth and development at the expense of the lamb, causing in-utero nutritional restriction. Ali et al. (2006) demonstrated an increase in birth weight of 25 grams for each one year increase in dam age. As in our study this effect was present until dams reached 4 years of age however the magnitude was smaller than the 230 – 430 g

117 increase between years seen in our study. This may be a reflection of the variation of the dam age effect between dam breeds as only Rambouilet ewes were used by Ali et al. (2006) whereas Afolayan et al. (2007) used crossbred ewes and the increase in lamb weight at birth between dam years was as much as 630g. While there was no dam age by dam breed effect in this study Rambouilet ewes were not used and the dam breeds which were used may not have been divergent enough to drive an effect. Dam age also influenced weaning and post weaning lamb weights with lambs from 4-6 year old dams tending to be heavier at 100 and 150 days. Increases in lamb weight at birth, weaning and post weaning (200 days) with increasing dam age were also found in the Afolayan

et al. (2007), study, although only 1-4 year old dams were included so the effect beyond

this age is unknown. At 240 days, when lambs were close to adult weight, this pattern was no longer evident. It is possible that the early influence on lamb weight is due to dam factors such as mothering ability, milk production and colostrum composition, the effects of which diminish post weaning, however these effects were not measured during this experiment.

The progeny of Terminal sires were heavier between birth and 240 days while the progeny of Merino sires were lighter at and had the slowest growth rates which is consistent with the findings of Fogarty et al. (2000). Furthermore, progeny of elite Terminal sires grew in excess of 250 g /day at 100, 150 and 240 days. As many nutritional requirement tables used for ration formulation only accommodate lamb growth rates up to 250 g / day (ARC, 1980) the nutrient requirements in the progeny of contemporary elite growth sires need to be updated.

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Within the progeny of Terminal sires, those from Border Leicester-Merino dams were heavier and had higher growth rates than those from Merino dams which is a reflection of the increased mothering ability (Alexander et al., 1983) and milk yield of Border Leicester-Merino dams (Kleemann et al., 1984).

Lamb weight was influenced by both production and genotypic factors and the magnitude of the effects varied (Table 4-5). Site, birth type-rear type and sire type had the greatest impact on lamb weight. Birth type had the largest effect on weight at birth and this was 1.8 times the magnitude of the effect across 1.8 units of the BWT breeding value. The difference in weight between sites had the second largest effect and was 1.3 times the magnitude of the BWT effect on birth weight. At 100, 150 and 240 days site had the largest effect on weight. At 100 days the differences between sites was 1.7 times the magnitude of the effect across 16 units of the WWT breeding value and between 1.1 and 6.6 times the magnitude of the other production effects. At 150 and 240 days the differences between sites was twice the magnitude of the effect across 23 units of the PWWT breeding value and between 1.2 and 9.2 times the magnitude of the other production effects.

Lamb growth rate was also influenced by both production and genotypic factors (Table 4-5). At 100 and 150 days site had the largest effect on growth rate whereas at 240 days year of birth had the largest effect. At 100 days there was no effect of WWT on growth rate however the difference in growth rate between sites was between 1.8 and 19.5 times the magnitude of the other production effects. At 150 days there was no effect of PWWT on growth rate however the difference in growth rate between sites and dam ages was similar and between 2.8 and 25.2 times the magnitude of the other production factors. At 240 days there was no main effect of PWWT and the difference in growth rates between years and sire types was similar and between 1.1 and 2.9 times the

119 magnitude of other production effects. Therefore, while sire ASBVs do impact on weight and growth rate in their progeny the effect is smaller than the impact of the environment, year of birth or birth type.

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4.5 Conclusion

This study demonstrated that both genotypic and production factors had significant effects on lamb growth. Selection for growth by selection of sires with an increased BWT, WWT and PWWT resulted in lambs that were heavier at birth, weaning and post weaning. The magnitude of these responses was moderated by nutrition hence varied between sites and with birth type-rear type combinations. Lambs born as multiples, with nutritional restriction from conception to weaning, had a reduced response to sire PWWT. Selection for leanness and muscling by selection of sires with a reduced PFAT and increased PEMD resulted had little impact on weight with increasing PEMD having no effect at all. Reducing PFAT increased lamb weight, however this effect was only present at the post weaning time point and was relatively small. As expected, single born lambs, male lambs and lambs born to Terminal sires were heavier and grew faster. Weight and growth rate were heavily influenced by environmental effects at each site and the effects were stable with minimal re-ranking of sites based on lamb weight over time. In conclusion, while selection for growth impacts on lamb growth, these effects and those of production factors such as sex and sire type are generally small compared to the impact of the environment.

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